Optical air data systems and methods

ABSTRACT

A method for remotely sensing air outside a moving aircraft includes generating laser radiation within a swept frequency range. A portion of the laser radiation is projected from the aircraft into the air to induce scattered laser radiation. Filtered scattered laser radiation, filtered laser radiation, and unfiltered laser radiation are detected. At least one actual ratio is determined from data corresponding to the filtered scattered laser radiation and the unfiltered laser radiation. One or more air parameters are determined by correlating the actual ratio to at least one reference ratio.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of priority to U.S. Provisional PatentApplication Ser. No. 60/699,630 filed Jul. 15, 2005. This application isalso a continuation-in-part of U.S. application Ser. No. 11/103,020filed 11 Apr. 2005, which is a continuation of U.S. application Ser. No.10/632,735 filed Aug. 1, 2003, now U.S. Pat. No. 6,894,768, which claimsbenefit of priority to U.S. Provisional Patent Application No.60/400,462 filed Aug. 2, 2002. All of the aforementioned applicationsare hereby incorporated by reference.

U.S. GOVERNMENT RIGHTS

This invention was made in part with the support of the U.S. Government;the U.S. Government has certain rights in this invention as provided forby the terms of Grant #NAS4-02043 awarded by the NASA Dryden FlightResearch Center.

BACKGROUND

An Air Data System (“ADS”) provides sensed telemetry informing pilots,navigators or Vehicle Management System computers of air parameter(s)affecting aircraft stability. These air parameters include, for example,air speed, air temperature and air pressure, each being useful fornavigation and flight control. The ADS exists in many forms, forexample, as mechanical, opto-mechanical or opto-electronic devices.

An Optical Air Data System (“OADS”) uses light to determine parametersof air speed. The OADS transmits light pulses into the atmosphere andreceives light that aerosols reflect or “backscatter” towards theaircraft. Aerosols are fine solids and/or liquid particles suspended inair or other gases. The OADS may also measure the Doppler effect byreceiving backscattered light and measuring its return frequency todetermine speed. Certain prior art OADSs rely on scattered light that isunpredictable because of aerosol distributions that vary significantlywith altitude and cloud content. In addition, some regions of theatmosphere contain too few aerosols to enable reliable air datameasurements, and such an OADS cannot determine air temperature or airpressure.

SUMMARY

In an embodiment, a method for remotely sensing air outside a movingaircraft includes generating laser radiation within a swept frequencyrange. A portion of the laser radiation is projected from the aircraftinto the air to induce scattered laser radiation. Filtered scatteredlaser radiation, filtered laser radiation, and unfiltered laserradiation are detected. At least one actual ratio is determined fromdata corresponding to the filtered scattered laser radiation and theunfiltered laser radiation. One or more air parameters are determined bycorrelating the actual ratio to at least one reference ratio.

In an embodiment, a method for remotely sensing air outside a movingaircraft includes generating laser radiation within a swept frequencyrange, wherein the swept frequency range includes at least twoabsorption features of at least one band stop filter. A portion of thelaser radiation is projected from the aircraft into the air to inducescattered radiation Filtered scattered laser radiation, filtered laserradiation, and unfiltered laser radiation are detected. A normalizedatmospheric return curve is determined from the filtered scattered laserradiation and the unfiltered laser radiation; a normalized filtertransmission curve is determined from the filtered laser radiation andthe unfiltered laser radiation. At least one actual ratio is determinedfrom the normalized atmospheric return curve. At least one referenceratio is determined from the normalized filter transmission curve and aRayleigh line shape corresponding to one or more estimated airparameters. One or more air parameters are determined by correlating theat least one actual ratio to the at least one reference ratio.

In an embodiment, a method for remotely sensing air outside a movingaircraft includes generating laser radiation within a swept frequencyrange. A portion of the laser radiation is projected from the aircraftinto the air to induce scattered radiation. Filtered scattered laserradiation, filtered laser radiation, unfiltered scattered laserradiation, and unfiltered laser radiation are detected. A normalizedfilter transmission and a normalized atmospheric return are determinedfrom the filtered scattered laser radiation, filtered laser radiation,unfiltered scattered laser radiation, and unfiltered laser radiation. Aplurality Doppler line shifts and a plurality of radial wind velocitiesare determined from a plurality of frequency shifts between thenormalized filter transmission and the normalized atmospheric return,wherein each frequency shift corresponds to an absorption feature of atleast one band stop filter.

In an embodiment, a system for sensing of air outside a moving aircraftincludes at least one laser for generating laser radiation and at leastone transceiver for projecting the laser radiation to the air and forreceiving scattered laser radiation from the air. Additionally, thesystem includes at least one band stop filter selected from the groupconsisting of a fixed frequency atomic vapor filter, an interferencefilter, a dichroic filter, a fiber Bragg grating filter, a Rugatefilter, and combinations thereof. Furthermore, the system includes acomputer for controlling the laser and for processing signals from thetransceiver to determine one or more air parameters based on thescattered laser radiation.

In an embodiment, a software product includes instructions, stored oncomputer-readable media, wherein the instructions, when executed by acomputer, perform steps for remotely sensing air outside a movingaircraft. The software product includes instructions for generatinglaser radiation within a swept frequency range. The software productalso includes instructions for determining a normalized atmosphericreturn curve from filtered scattered laser radiation and unfilteredlaser radiation and instructions for determining a normalized filtertransmission curve from filtered laser radiation and the unfilteredlaser radiation. Furthermore, the software product includes instructionsfor determining at least one actual ratio from the normalizedatmospheric return curve and instructions for determining at least onereference ratio from the normalized filter transmission curve and aRayleigh line shape corresponding to one or more estimated airparameters. Also included within the software product are instructionsfor determining one or more air parameters by correlating the at leastone actual ratio to the at least one reference ratio.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows one Optical Air Data System (“OADS”), according to anembodiment.

FIG. 2 shows one OADS, according to an embodiment.

FIG. 3 shows one graph useful in illustrating an exemplary air speedcalculation with an OADS, according to an embodiment.

FIGS. 4-7 show graphs illustrating exemplary calculations for other airparameters with an OADS, according to an embodiment.

FIG. 8 is a flowchart showing one exemplary method of operation of anOADS, according to an embodiment.

FIG. 9 is a flowchart showing one exemplary method of operation of anOADS, according to an embodiment.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shows one Optical Air Data System (“OADS”) 101 mounted on orwithin an aircraft 102. In this embodiment, OADS 101 is configured forprojecting laser radiation 103 to air 104. Laser radiation 103 impingeson air 104 and aerosol particles 105 (in air 104), causing scattering oflaser radiation 103, which is represented in FIG. 1 as a scatter field106. Distance between aircraft 102 and scatter field 106 is controlledby overlap between laser radiation 103 and the transceiver 110 field ofview at a distance from aircraft 102, to provide an optimized intensityfor return laser radiation 107 and to eliminate possible measurementerror arising from displaced air proximate to aircraft 102. OADS 101detects backscattered laser radiation 107 that is backscattered from air104 at laser scatter field 106. Radiation 107 may be in the ultra-violet(UV) spectrum, for example, having a wavelength within a range of 250 nmto 270 nm; however, other ranges may alternatively be used to producescatter field 106.

Return laser radiation 107 typically contains molecular scattered (e.g.,Rayleigh) components 107A and/or aerosol scattered (e.g., Mie)components 107B. OADS 101 distinguishes the molecular scatteredcomponents 107A from the aerosol scattered components 107B andcorrespondingly determines one or more air parameters based onbackscattered laser radiation 107. Examples of such air parametersinclude air speed, air pressure, air temperature and/or aircraftorientation angles relative to the local wind. OADS 101 may beconfigured with other aircraft as well, such as unmanned air vehicles(UAVs), helicopters, gliders and space shuttles. Although illustratedwithin a “nose” 108 of aircraft 102, OADS 101 may be configured in anyother part of aircraft 102.

As shown in FIG. 1, OADS 101 includes a laser 109 configured forgenerating laser radiation 103. Transceiver 110 is configured fortransmitting laser radiation 103, from laser 109 via optical coupling111, and receiving backscattered laser radiation 107. Optical coupling111 may exist in the form of a fiber optic connection or free spacetransmission. Accordingly, transceiver 110 projects the laser radiationas laser radiation 103 to air 104. Air 104 scatters laser radiation 103at scatter field 106 in a plurality of directions (e.g., illustrated asvectors 112). Scatter field 106 also returns, or backscatters, radiation107 towards transceiver 110, which subsequently receives thebackscattered laser radiation 107. Transceiver 110 convertsbackscattered laser radiation 107 to processable electronic signals, viacomputer 113, to determine the air parameters.

Computer 113 communicatively couples with transceiver 110 and processessignals from transceiver 110 to distinguish a molecular scatteredcomponent 107A from an aerosol scattered component 107B. Computer 113determines the air parameters based on laser radiation 107 backscatteredfrom molecules and/or aerosols in air 104. Accordingly, as describedbelow, computer 113 may employ one or more digital signal processingalgorithms to determine such parameters.

While OADS 101 illustrates one transceiver 110 in an exemplaryembodiment, a plurality of transceivers may be used, depending on anapplication. For example, a helicopter employing OADS 101 may use twotransceivers 110 to determine air parameters such as a forward velocity(e.g., air speed) and a horizontal plane, or “yaw”, of the helicopter.An airplane may use three transceivers 110 positioned in a particularmanner to determine various aircraft geometries, such as angle of attackand sideslip, in addition to the air parameters of air speed, airpressure and air temperature. In addition, air vehicles (fixed wing androtary) may employ three or more transceivers and/or lasers to increaseOptical Air Data System reliability through a redundant systemarchitecture. Using three OADS transceivers mounted orthogonally to oneanother may fully resolve a total airspeed vector by providing threeindependent measurements for the air speed vector (i.e., correspondingto three axes of a Cartesian coordinate system). The transceivers arefor example located in uncommon planes and their geometry respective ofa known aircraft center-line. Vector algebra may then be used todetermine the full airspeed vector, including forward air speed,angle-of-sideslip and angle-of-attack.

FIG. 2 shows one OADS 140. OADS 140 illustrates another embodiment usedfor determining air parameters, such as those described in FIG. 1, basedupon laser radiation backscattered from both air molecules and aerosols.In this embodiment, OADS 140 includes laser 141 configured forgenerating laser radiation 142. Laser 141 may be a tunable laser havinga tuned center wavelength of about 253.7 nm, although other wavelengthsmay be used. For example, laser 141 may be a frequency quadrupled,Nd:YAG (i.e., neodymium:yttrium-aluminum-garnet) pumped Ti:Sapphire(titanium-sapphire) laser. Alternatively, frequency-quadrupled Yb-doped(ytterbium-doped) fiber lasers may be used that offer important benefitsof smaller size, lighter weight, increased robustness and improvedreliability, as compared to Nd:YAG-pumped Ti:Sapphire lasers. Laser 141may generate laser radiation that is tunable across a frequency range ofabout 40 GHz; laser 141 may be a continuous wave laser which sweeps infrequency across this range, or it may be a pulsed laser controlled suchthat each pulse has a frequency distribution centered about a tunablepeak frequency. In one embodiment, the peak frequency increments byabout 100 MHz from each pulse to the next. Laser 141 may tune ±20 GHzabout a center frequency of approximately 1182.5 THz, or c/253.7 nm,where c is the speed of light (approximately 3×10⁸ M/s). In theillustrated embodiment, laser 141 radiates laser radiation 142 to beamsplitter 143, which splits the beam into two components, 143A and 143B.Component 143A is directed through air 144; component 143B is directedto beam splitter 145.

In particular, component 143A of laser radiation 142 directed to air 144is scattered into scatter field 146. Scattering of component 143A isillustrated by scattering vectors 147 in scatter field 146, whereasreturn scattering is illustrated by backscattered laser radiation 148.Component 143B of the laser radiation 142 is used as a reference forcomparison to backscattered laser radiation 148. Such a comparison isfor example useful in determining air parameters such as air speed,since transmitted and received frequencies of the laser radiation may beascertained for use in a Doppler equation; such a process is explainedin greater detail herein below.

In the illustrated embodiment, backscattered laser radiation 148 isreceived through optics 149. In one example, optics 149 is a telescopethat gathers backscattered laser radiation 148 into a beam 150. Optics149 also directs beam 150 to beam splitter 151, to split beam 150 intotwo components 150A/150B. Component 150B of beam 150 passes throughvapor filter 152 to detector 153 to produce electronic signal 158representative of the component 150B impinging detector 153; whereascomponent 150A is directed by beam splitter 151 to detector 154.

In one embodiment, detector 154 is a photodetector that receivesradiation 150A and converts it into an electronic signal 155. Detector154 connects to a central computer 156 to process electronic signal 155.Similarly, detector 153 is a photodetector configured for detectingcomponent 150B, which is filtered by vapor filter 152 as filteredcomponent 157. Detector 153 converts component 157 to an electronicsignal 158 for processing by central computer 156.

Accordingly, electronic signal 158 corresponds to backscattered laserradiation 148 as filtered by vapor filter 152; and electronic signal 155corresponds to unfiltered backscattered laser radiation 150A. Electronicsignal 155 is thus used to nullify certain anomalies as computer 156processes electronic signal 158. For example, when processed withelectronic signal 158, signal 155 may be used to remove, from signal158, certain laser transmission power fluctuations in filtered component157 caused by atmospheric changes in air 144. Such a process isexplained in more detail in connection with FIGS. 4-7.

Computer 156 includes lookup tables 170 and 172 that may be utilized todetermine temperature and/or pressure as discussed below.

Reference component 143B of the laser radiation 142 is split into twocomponents 159 and 160 by beam splitter 145. Component 160 is directedby beam splitter 145 to vapor filter 152 via mirrored surface 161, tomeasure filter characteristics, whereas component 159 is directed bybeam splitter 145 to detector 162, to generate electronic signal 163.Electronic signal 163 is for example used to normalize powerfluctuations in the return of backscattered laser radiation 148 causedby power fluctuations in the generation of laser radiation 142 by laser141. Such a process is explained in more detail in FIGS. 4-7.

Vapor filter 152 filters component 160 to produce filtered component164. Filtered component 164 is directed to detector 165, via mirroredsurface 166, and then converted to an electronic signal 167. Centralcomputer 156 processes electronic signal 167 to determine filtercharacteristics, such as frequencies and suppression features of theband stop region of vapor filter 152. One such process is also explainedin more detail in context of FIGS. 4-7.

It should be noted that while FIG. 2 shows OADS 140 as having free spaceoptical transmission and optical components such as beam splitters 143,145 and 151 and mirrors 161 and 166, optical fiber may be used for laser141 transmission along paths 142, 143A, 143B, 159, 160, 164, 150, 150A,150B and/or 157; in such an embodiment, fiber splitters may be used inplace of beam splitters 143, 151 and 145, and mirrors 161 and/or 166 maybe eliminated.

It will also be appreciated that although the embodiment shown in OADS140 of FIG. 2 employs vapor filter 152, other types of filters may beutilized. For example, notch or band-stop filters such as interferencefilters, dichroic filters, fiber Bragg grating filters and/or Rugatefilters may be utilized. A filter used in place of vapor filter 152 mayadvantageously have properties such as: (1) high optical absorptionwithin a stop-band region on the order of 40-60 dB or more; (2) a notchfilter absorption width between about 5 GHz and 40 GHz, with anabsorption width under 10 GHz being preferred; and (3) steep absorptionsidewalls, with a 10% -90% absorption transition occurring within about5 GHz or less. Pass-band filters may also be used, such as when theyoperate in a reflection mode such that a reflection produces a stop-bandfilter. Single filters with multiple absorption features may beutilized, or optical or fiber splitters may be used to route opticalsignals through multiple filters, each filter having a single absorptionfeature.

Filters other than atomic vapor filters may provide certain advantages.For example, while the absorption frequencies of atomic vapor filtersare reliably tied to properties of an atomic vapor used, their use mayconstrain an OADS to include a tunable laser having output at suchfrequencies. However, certain tunable lasers may have improvedperformance and/or stability at frequencies that do not convenientlymatch atomic vapor filter absorption frequencies. Certain filters suchas interference filters, dichroic filters, fiber Bragg grating filtersand/or Rugate filters may be designed to have absorption features tunedto a preferred frequency output range of a tunable laser, rather thantuning the laser to the filter. The use of a tunable laser selected onits merits, and a matching notch filter in an OADS may thus (1) enableuse of higher laser output power for improved return signal strength,(2) make the OADS more robust with respect to thermal stability,vibration and shock, (3) eliminate hazardous materials (e.g., mercury)from the OADS, and/or (4) reduce size, weight and/or cost of the OADS.

FIG. 3 shows one graph 200 useful in illustrating an exemplary air speedcalculation with OADS 140. Graph 200 shows two curves, 201 and 202,comparing normalized laser radiation magnitudes as a function offrequency (signal strength, that is, normalized laser radiationmagnitude, is plotted with respect to axis 205, and frequency is plottedwith respect to axis 204). Curve 202 exemplifies filtered radiated laserradiation such as that of filtered component 164 of FIG. 2. As such,curve 202 shows filter characteristics of vapor filter 152 of FIG. 2determined by processing of electronic signal 167. Curve 202 shows apeak absorption of filter 152 occurring at a down-translated frequencyof 0 GHz. By way of example, the actual peak absorption frequency offilter 152 may be about 1182.5 THz (i.e., having a correspondingwavelength of about 253.7 nm).

Laser radiation 142 generated by laser 141 passes through filter 152 toprovide filtered component 164. Once filtered component 164 is convertedto electronic signal 167 by detector 165, computer 156 analyzes andstores features of vapor filter 152 through digital signal processing ofsignal 167 (e.g., computer 156 stores reference features, obtained undercontrolled conditions, for use in future calculations). As shown in thisexample, features of vapor filter 152 have approximately 10% normalizedabsorption at approximately ±5 GHz (i.e., 0.9 normalized transmissionfactor at approximately ±5 GHz according to axis 205) about the peakabsorption frequency. Other types of suitable filters may includedifferent absorption/transmission features.

Curve 201 exemplifies filtered backscattered laser radiation such asthat of filtered component 157 of FIG. 2. In one embodiment, curve 201is used to determine air speed by comparison to curve 202. For example,curve 202 illustrates how vapor filter 152 affects laser radiation 142;curve 201 similarly illustrates how vapor filter 152 affects laserradiation 142 as laser radiation 142 is backscattered (e.g., returns asradiation 148) from air 144. Frequency shift 203 represents the changein frequency of peak absorption for vapor filter 152 between transmittedlaser radiation 142 and returned laser radiation 148. Computer 156processes algorithms applying Doppler velocity equation to determine airspeed from frequency shift 203.

To determine air speed in one embodiment, computer 156 determines howfar in frequency the peak absorption frequency of filtered component 157has shifted from the initial laser frequency by comparing curve 202 tocurve 201 (e.g., comparing peak absorption frequencies of filteredcomponents 157 and 164). Frequency shift 203 substantially equates to aradial wind velocity through the Doppler velocity equation:$\begin{matrix}{{{\Delta\quad v_{D}} = \frac{2V_{R}}{\lambda}},} & \left( {{Eq}.\quad 1} \right)\end{matrix}$where Δv_(D) represents the Doppler frequency shift, V_(R) representsvelocity component of the vehicle (e.g., aircraft 101 of FIG. 1) alongthe laser direction of propagation 143A and λ represents the wavelengthof laser radiation 142.

In one embodiment, wind velocity component V_(R) may be measured bydetermining the frequency shift from curve 202 of graph 200 as comparedto curve 201 of graph 200. This is accomplished by calculating asymmetry point of each curve 201 and 202 and determining a difference insymmetry points between the two curves.

Vapor filter 152 may have a plurality of absorption features.Consequently, OADS may have a plurality of absorption maxima, such asthose illustrated by curves 201 and 202 of FIG. 3, which may be used toprovide a more accurate estimate of the vehicle's velocity. Thevehicle's velocity, V_(R), may be calculated using equation 1 for eachabsorption feature. An average velocity of the vehicle may then becalculated from each value of V_(R).

FIGS. 4-7 show graphs illustrating exemplary calculations for other airparameters with OADS 140. For example, after determining frequency shiftdue to air speed as shown in FIG. 3, other air parameters such as airtemperature and air pressure may be calculated. In one example, computer156 initially determines an intensity measurement of the detectedbackscattered laser radiation (e.g., filtered component 157 detected bydetector 153) from electronic signal 158. This experimentally verifiedintensity measurement of returned laser radiation corresponds to thefollowing equation:S _(S)(v)=P _(L) T _(L) D _(S) T _(R) E _(S) ∫dv _(r) ∫dv _(laser) [L(v_(laser))F(v _(r) −v)(r R(v _(r)−(v _(laser) −Δv _(D)))+mM(v_(r)−(v_(laser) −Δv _(D))))]  (Eq. 2)where S_(S)(v) is electronic signal 158 from detector 153; P_(L) is thelaser power, T_(L) is the transmission coefficient through air 144 alonglaser path 143A, L(v_(laser)) is the laser line shape inherent to thelaser 141 output as a function of laser frequency v_(laser), T_(R) isthe transmission coefficient through air 144 along laser path 148, E_(S)is optical efficiency of the detector channel through detector 153, F(v)is the band stop frequency range of vapor filter 152 centered at afrequency of v, R is Rayleigh scattering as a function of frequency(applicable to the Rayleigh regime) v_(r) for backscattered laserradiation minus the quantity of laser frequency v_(laser) minus theDoppler shift Δv_(D), r is the Rayleigh scattering magnitude coefficientdependent on air density and the Rayleigh backscattering coefficient, Mis Mie scattering as a function of v_(r) minus the quantity of v_(laser)minus Δv_(D), m is the Mie scattering magnitude coefficient dependent onaerosol concentration and the Mie backscattering coefficient, and D_(S)is detector 153 efficiency. The Rayleigh backscattering coefficient rand the Mie backscattering coefficient m are constant for a particularatmosphere. These coefficients correspond to the number of scatterers(i.e., molecules for Rayleigh, aerosols for Mie) per unit volume ofatmosphere.

Next, computer 156 may determine other air parameters, utilizing theresult obtained for the measured intensity of the returned laser energy.Such a process, for example, may begin by determining characteristics ofvapor filter 152 by transmitting of reference laser radiation 160through vapor filter 152. For example, measuring band stopcharacteristics of vapor filter 152 with laser 141 (e.g., via component143B to electronic signal 167) during experimentation yields aconvolution of the laser wavelength and the filter according to thefollowing equation:S _(F)(v)=P _(L) E _(F) D _(F) ∫dv _(laser) [L(v _(laser))F(v _(laser)−v)], (Eq. 3)where S_(F)(v) is signal 167 from detector 165 as a function offrequency v (e.g., as illustrated in curve 221 of FIG. 4); E_(F) is theoptical efficiency of filter 152 collection along paths 160 and 164, andD_(F) is detector 165 efficiency.

Note that all optical efficiencies E_(F) and E_(S) capture signal lossesthat are optical in nature. For example, E_(F), the optical efficiencyfor detector 165, includes the optical beam splitting ratios for beamsplitters 143 and 145, the optical transmission and coupling acrossfilter 152 and the optical delivery efficiency onto detector 165. E_(S),the optical collection efficiency for detector 153, includes thecollection efficiency of telescope 149, the optical coupling efficiencyinto path 150, the beam splitter ratio of beam splitter 151, thetransmission efficiency across filter 152 and the delivery efficiencyonto detector 153. Detector efficiencies D_(F) and D_(S) include thedetector conversion efficiencies for detectors 165 and 153,respectively. Thus, D_(F) is the conversion efficiency whereby detector165 converts laser radiation along path 164 into an electrical signal167. Likewise, D_(S) is the conversion efficiency whereby detector 153converts laser radiation along path 157 into an electrical signal 158.

Backscattered laser radiation 148 may include power fluctuations thatare caused by laser 141 while generating laser radiation 142.Accordingly, laser radiation detected by detector 162 (e.g., viacomponent 159) may be utilized to normalize power fluctuationsattributable to laser 141. In one embodiment, detector 162 convertscomponent 159 into electronic signal 163. In turn, computer 156processes and normalizes according to the following equation:S _(L)(v)=P _(L) E _(L) D _(L) ∫dvL(v),   (Eq. 4)where S_(L)(P) is the electronic signal 163 from detector 162, E_(L) isthe optical collection efficiency for detector 162, D_(L) is theconversion efficiency of detector 162 and P_(L) is the power of laser141. Note that the optical collection efficiency E_(L) includes the beamsplitting ratios of beam splitters 143 and 145 and the deliveryefficiency of laser beam path 159 onto detector 162.

Curve 221 of graph 220 of FIG. 4 represents the magnitude of laserradiation (component 164) filtered by vapor filter 152 and normalizedbetween 0 and 1. Curve 221 represents the magnitude of the laserradiation as a function of frequency (i.e., laser radiation magnitudeplotted with respect to axis 222 and frequency plotted with respect toon axis 223). Curve 221, therefore, illustrates filtered laser radiationvia component 160 as determined by computer processing of electronicsignal 167, plotted as laser radiation magnitude normalized between 0and 1, versus frequency.

In one embodiment, absorption/transmission characteristics of vaporfilter 152 are normalized using Eq. 3 and Eq. 4. Eq. 3 yields stop bandcharacteristics of filter 152 and Eq. 4 accounts for power fluctuationsin the generation of laser radiation 142. With the power fluctuations ofEq. 4 substantially removed, a “normalization channel” is created, andpower fluctuations attributable to atmospheric changes may be accountedfor.

In one embodiment, additional power fluctuations caused by atmosphericchanges in air 144 are also removed. For example, laser radiationdetected by detector 154 (e.g., via component 150A) assists in removinglaser power fluctuations caused by atmospheric changes in air 144.Accordingly, detector 154 converts received laser radiation intoelectronic signal 155. Computer 156, in turn, processes electronicsignal 155 to determine the normalized laser radiation magnitudeaccording to the following equation:S _(N) =P _(L) T _(L) T _(R) E _(N)D_(N) ∫dv∫dv _(laser)[L(v_(laser))(rR(v−(v _(laser) −Δv _(D)))+mM(v−(v _(laser) −Δv_(D))))]  (Eq. 5)where S_(N) is the signal 155 from detector 154; E_(N) is opticalcollection efficiency of the detector 154 and D_(N) is the conversionefficiency of detector 154.

In one embodiment, it is advantageous to normalize the variouscharacteristic functions to enable a closed-loop solution to the processof determining temperature and pressure. In one example, therefore,computer 156 calculates the normalized laser line shape according tofollowing equation:∫L(v _(laser))dv _(laser)=1,   (Eq. 6)where (as before) v_(laser) is laser line shape frequency and L denotesthe laser line shape as a function of frequency. In another example,computer 156 calculates normalized Rayleigh Function according to thefollowing equation:∫R(v _(r))dv _(r)=1,   (Eq. 7)where R denotes the Rayleigh line shape as a function of frequencyv_(r), applicable to the Rayleigh regime. In another example, computer156 scales the electronic signal 167 recorded from detector 165 bydividing all recorded values by the maximum value according to thefollowing equation:MAX(S _(F)(v))=1,   (Eq. 8)where MAX denotes an operation that finds a maximum value of aparticular function, and S_(F) denotes electronic signal 167 measuredfrom detector 165, as a function of frequency v (e.g. as illustrated incurve 221 of FIG. 4). In another example, computer 156 normalizes theMie Function according to the following equation:M(v)=δ(v),   (Eq. 9)where δ(v) is the delta function.

In one embodiment, dividing the signal 167 collected from detector 165(and represented by Eq. 3, above) by the signal 163 collected fromdetector 162 (and represented by Eq. 4, above) removes laser 141 powerfluctuations, as follows: $\begin{matrix}{\frac{S_{F}(v)}{S_{L}(v)} = \frac{P_{L}E_{F}D_{F}{\int{\mathbb{d}\quad{v_{laser}\left\lbrack {{L\left( v_{laser} \right)}{F\left( {v_{laser} - v} \right)}} \right\rbrack}}}}{P_{L}E_{L}D_{L}{\int{{\mathbb{d}v}\quad{L(v)}}}}} & \left( {{Eq}.\quad 10} \right)\end{matrix}$

Equation 10 simplifies to: $\begin{matrix}{\frac{S_{F}(v)}{S_{L}(v)} = {\frac{E_{F}D_{F}}{E_{L}D_{L}}{{LF}(v)}}} & \left( {{Eq}.\quad 11} \right)\end{matrix}$

where LF(p) represents a convolution of functions L and F (that is, afunction that represents the effects of functions L and F combined ateach frequency v).

In one embodiment, tuning the laser 141 to a reference frequency v_(ref)far enough removed from the effects of the vapor filter 152 enables themeasurement of the ratio of the optical and detector efficiencies of thesignal channels 167 (S_(F), represented by Eq. 3 above) and 163 (S_(L,),represented by Eq. 4 above). This, in turn, enables the normalization ofthe signal 167 measurement to one, for simultaneously checking forlaser, detector and filter abnormalities on a scan-by-scan basis:$\begin{matrix}{\frac{S_{F}\left( v_{ref} \right)}{S_{L}\left( v_{ref} \right)} = \frac{E_{F}D_{F}}{E_{L}D_{L}}} & \left( {{Eq}.\quad 12} \right)\end{matrix}$

In one embodiment, LF(v) are determined to generate a look up table ofthe convolution of theoretical Rayleigh functions (calculated in termsof temperature and pressure) with the measured filter function. Sincethe measured filter function is already the convolution of the laser andfilter spectra, convolving the Rayleigh function with the measuredfilter signal 167 yields the expected return signal from an atmosphereof pure Rayleigh scatterers.

In one embodiment, the measured signal 158, which is the backscatterreturn from the atmosphere 144 that passes through the vapor filter 152(and is represented by Eq. 2 above), is divided by the signal 155, whichis the backscatter return from the atmosphere 144 that does not passthrough vapor filter 152 (and is represented by Eq. 5 above). Thiscalculation removes changes in signal transmission that are independentof the factors to be measured: $\begin{matrix}{\frac{S_{S}(v)}{S_{N}(v)} = \frac{\begin{matrix}{P_{L}T_{L}D_{S}T_{R}E_{S}{\int{{\mathbb{d}v}{\int{\mathbb{d}v_{laser}}}}}} \\\begin{bmatrix}{\left( {{L\left( v_{laser} \right)}{F\left( {v_{r} - v} \right)}} \right)\left( {{rR}\left( {v_{r} - \left( {v_{laser} -} \right.} \right.} \right.} \\\left. {\left. \left. {\Delta\quad v_{D}} \right) \right) + {{mM}\left( {v_{r} - \left( {v_{laser} - {\Delta\quad v_{D}}} \right)} \right)}} \right)\end{bmatrix}\end{matrix}}{\begin{matrix}{P_{L}T_{L}T_{R}E_{N}D_{N}{\int{{\mathbb{d}v}{\int{\mathbb{d}v_{laser}}}}}} \\\begin{bmatrix}{{L\left( v_{laser} \right)}\left( {{{rR}\left( {v_{r} - \left( {v_{laser} - {\Delta\quad v_{D}}} \right)} \right)} +} \right.} \\\left. \left. {{mM}\left( {v_{r} - {\Delta\quad v_{D}}} \right)} \right) \right)\end{bmatrix}\end{matrix}}} & \left( {{Eq}\quad 13} \right)\end{matrix}$Since M is a delta function, Equation 13 simplifies to: $\begin{matrix}{\frac{S_{S}(v)}{S_{N}(v)} = {\left\lbrack \frac{E_{S}D_{S}}{E_{N}D_{N}} \right\rbrack\frac{{{rLFR}\left( {v_{laser} - {\Delta\quad v_{D}}} \right)} + {{mLF}\left( {v_{laser} - {\Delta\quad v_{D}}} \right)}}{r + m}}} & \left( {{Eq}.\quad 14} \right)\end{matrix}$

where LFR(P) represents a convolution of functions L, F and R in thesense of the convolution LF(v) discussed above.

In one embodiment, tuning laser 141 to reference frequency v_(ref) farenough removed from the effects of the vapor filter 152 enables themeasurement of the ratio of the optical and detector efficiencies of thesignal channels 158 (S_(S) as represented by Eq. 2 above) and 155 (S_(N)as represented by Eq. 5 above). This enables a check for abnormalitiesin the filter on a scan-by-scan basis: $\begin{matrix}{\frac{S_{S}\left( v_{ref} \right)}{S_{N}\left( v_{ref} \right)} = \frac{E_{S}D_{S}}{E_{N}D_{N}}} & \left( {{Eq}.\quad 15} \right)\end{matrix}$

In one embodiment, a variable K_(ref) may be defined as: $\begin{matrix}{K_{ref} = \frac{S_{S}\left( v_{ref} \right)}{S_{N}\left( v_{ref} \right)}} & \left( {{Eq}.\quad 16} \right)\end{matrix}$

Once both data sets (i.e., S_(S) and S_(N)) are symmetric about the samedata point, computer 156 calculates temperature and pressure from thereturn signal. Initially, computer 156 uses theoretical Rayleighfunctions that are functions of temperature and pressure in conjunctionwith the measured filter transmission to generate a lookup table 170that stores laser, Rayleigh, and filter (LFR(v)) convolutions that aredependent on atmospheric temperature and pressure. Computer 156 may thencompare a normalized return signal to a value stored in lookup table 170to determine atmospheric temperature and pressure. In order to comparethe return signal with the lookup table 170, computer 156 accounts forthe magnitude of Mie scatterers as well as any changes in air densitythat may change the magnitude of the Rayleigh signal.

A vapor filter may be used as a bandstop filter; such filters typicallyprovide frequency stability, optical depth, and optimal filter shape.For the purposes of separating the Rayleigh and Mie scattering, anoptical depth of approximately 60 dB provides excellent absorption ofMie scattering within a small frequency variance around v₀ (i.e., wherev_(f) is a normalized frequency of 0 GHz). For example, an atomic vaporfilter may provide 60 dB of absorption in a frequency region that is notcontaminated by Mie scattering. This region may be used in acquiringinitial estimates of pressure and temperature (explained below in FIG.5). Such absorption is observable in FIG. 5 below as the measured signalS_(F) which has the magnitude of zero centered about v₀. This dataprovides information about pure Rayleigh scattering that may be used tocalculate the ratio of Mie scattering to Rayleigh scattering, as shownin Eq. 17: $\begin{matrix}{\frac{S_{S}\left( v_{0} \right)}{S_{N}\left( v_{0} \right)} = {\left\lbrack \frac{E_{S}D_{S}}{E_{N}D_{N}} \right\rbrack\frac{{{rLFR}\left( v_{0} \right)} + {{mLF}\left( v_{0} \right)}}{r + m}}} & \left( {{Eq}.\quad 17} \right)\end{matrix}$Since the vapor filter fully attenuates the Mie scattering in thisregion: $\begin{matrix}{{\frac{S_{S}\left( v_{0} \right)}{S_{N}\left( v_{0} \right)} = {\left\lbrack \frac{E_{S}D_{S}}{E_{N}D_{N}} \right\rbrack\frac{{rLFR}\left( v_{0} \right)}{r + m}}},} & \left( {{Eq}.\quad 18} \right)\end{matrix}$where LFR(v₀) is the value of the theoretical return signal atparticular atmospheric temperature and pressure. Accordingly, computer156 calculates the ratio of Mie scattering by first defining a variableK₀ as follows: $\begin{matrix}{K_{0} = \frac{S_{S}\left( v_{0} \right)}{S_{N}\left( v_{0} \right)}} & \left( {{Eq}.\quad 19} \right)\end{matrix}$and then solving for the ratio $\begin{matrix}{\frac{m}{r} = {{\frac{K_{0}}{K_{a}}{{LFR}\left( v_{0} \right)}} - 1}} & \left( {{Eq}.\quad 20} \right)\end{matrix}$Using the normalized signal return in the region of interest (i.e., thesloped region between the minimum and maximum of the signal return) andwriting the result in terms of the ratio of m over r, yields thefollowing: $\begin{matrix}{\frac{S_{S}(v)}{S_{N}(v)} = {K_{a}\frac{{{LFR}(v)} + {\frac{m}{r}{{LF}(v)}}}{1 + \frac{m}{r}}}} & \left( {{Eq}.\quad 21} \right)\end{matrix}$Substituting the ratio of m and r of Eq. 20 into Eq. 21 yields:$\begin{matrix}{\frac{S_{S}(v)}{S_{N}(v)} = {{K_{a}\frac{{LFR}(v)}{{LFR}\left( v_{0} \right)}} + {{{LF}(v)}\left\lbrack {1 - \frac{K_{0}}{K_{a}{{LFR}\left( v_{0} \right)}}} \right\rbrack}}} & \left( {{Eq}.\quad 22} \right)\end{matrix}$Solving for LFR(v) yields: $\begin{matrix}{{{{LFR}(v)} = {{\frac{S_{S}(v)}{S_{N}(v)}\frac{{LFR}\left( v_{0} \right)}{K_{a}}} + {{{LF}(v)}\left\lbrack {\frac{1}{K_{a}} - \frac{{LFR}\left( v_{0} \right)}{K_{0}}} \right\rbrack}}},} & \left( {{Eq}.\quad 23} \right)\end{matrix}$where the measured signal return LFR(v) is written in terms of measuredquantities and the theoretical values of LFR(v₀). Computer 156 thencalculates LFR(v) and compares it to the lookup table 170 to determineatmospheric temperature and pressure, described in greater detail inFIG. 5.

Accounting for power fluctuations, optical efficiencies and detectorefficiencies as described herein allows for an independent check onvapor filter 152 while OADS 140 operates. With variable characteristicsof detector channels and power fluctuations accounted for, computer 156may determine, for example, the substantially constant characteristicsof vapor filter 152, such that more accurate measurements of receivedbackscattered laser radiation (e.g., laser radiation 148) are obtained.

In one embodiment, the normalization channel depicted in FIG. 4 is usedto remove atmospheric power fluctuations of laser radiation 148. Indoing so, computer 156 measures Rayleigh and Mie components of laserradiation 147 in terms of optical efficiencies and detectorefficiencies. Such efficiencies are typically measured on a shot-by-shotbasis during the analysis process. In an exemplary embodiment ofoperation, laser 141 generates and transmits laser radiation 142 as aseries of pulses at a particular pulse repetition frequency (“PRF”),while in other embodiments laser 141 is a continuous wave laser (asdiscussed in connection with FIG. 2). Computer 156 then measures theRayleigh and Mie components in terms of optical efficiencies anddetector efficiencies on a pulse-by-pulse basis.

To measure Rayleigh components and Mie components, in one embodiment,OADS 140 tunes the frequency of the laser radiation 142 transmitted bylaser 141. For example, laser 141 transmits the laser radiation 142 atdistal frequencies from the peak absorption frequency of filter 152(illustrated by v_(f) in FIG. 4) to provide a frequency-independentmeasurement. Computer 156 then determines the line shape of laserradiation 142 through filter 152.

In one embodiment, measured intensity of the detected backscatteredlaser radiation (e.g., as determined by electronic signal 158) isfunctionally compared to normalized atmospheric factors. The measuredintensity often depends upon Mie scatterers (e.g., aerosols) and airdensity changes due to altitude changes and temperature changes. The airdensity changes and the temperature changes are not, however, removedthrough the normalization processes described herein. For computer 156to accurately determine air parameters such as temperature and pressureof air 144, air density changes are removed from the detectedbackscattered laser radiation so that computer 156 may accuratelydetermine the air parameters.

FIG. 5 shows graph 240 with curves 241 (detected backscattered laserradiation at a higher air density causing both Rayleigh and Miescattering), 242 (detected backscattered laser radiation at an airdensity causing Rayleigh scattering) and 243 (normalized Rayleighscattering). Curves 241, 242 and 243 illustrate laser radiationmagnitudes (plotted with respect to axis 250) as a function of frequency(plotted with respect to axis 251). In one embodiment, computer 156processes data from curves 241, 242 and 243 to determine other airparameters. For example, Mie scattering effects are substantiallyisolated and removed from calculations to determine air temperature andair pressure, since these Mie scattering effects produce inaccuratemeasurements due to inconsistent aerosol concentrations.

In one embodiment, to determine the air temperature and air pressure,computer 156 processes the data from curves 241, 242 and 243 tosubstantially isolate and remove the Mie scattering effects, such asthose found in curve 241. In processing the data from curves 241, 242and 243, computer 156 calculates lookup table 170 in substantially realtime using a measured laser/filter profile (i.e., as measured atdetector 165 of FIG. 2) convolved with theoretical Rayleigh functionsfor a particular temperature and pressure (e.g., illustrated by curves242 and 243). Computer 156 then scales the measured return signal LFR(v)(i.e., illustrated by curve 241 in this example) with the ratio of m tor determined by Eq. 20. Computer 156 then analyzes data near the deepestportion of the filter attenuation (i.e., approximately ±0.5 GHz fromv_(f)) to estimate pressure and/or temperature. This portion correspondsto a 60 dB region of absorption that is not contaminated by Miescattering. Use of this region is a preferred aspect of the calculationtechnique that provides temperature and pressure accuracy by providing areliable temperature base from which to increment temperature and/orpressure estimates.

Computer 156 calculates theoretical Rayleigh return assuming an initialtemperature estimate and performs a Least Square Error (LSE) calculationto determine the accuracy of the temperature with respect to thetheoretical Rayleigh function. Computer 156 repeats the process withincremental changes to temperature and/or pressure until an optimal fit(i.e., an LSE calculation that corresponds to design specifications) isachieved. Although discussed in detail with respect to LSE, otherapproximation methods, such as Newton-Raphson and Monte Carlo, may beused in alternative embodiments. Accordingly this disclosure teaches byway of example and not by limitation.

Temperature affects air density in a manner that is reciprocal topressure; increasing pressure increases density, while increasingtemperature decreases density. Additionally, increasing temperatureincreases the Rayleigh lineshape width while increasing pressureincreases the Rayleigh lineshape height. Accordingly, for eachincremental value of temperature and/or pressure, the Rayleigh lineshapeis unique. Such scattering theory is discussed in “On The Kinetic ModelDescription Of Rayleigh-Brillouin Scattering From Molecular Gases”, G.C. Tenti, D. Boley and R. C. Desai, Canadian Journal of Physics, vol.52, pg. 285-290 (1974).

In one example, computer 156 determines air density changes by aligningpeak absorption frequencies of curves 241, 242 and 243, illustrated atfrequency v_(f). Since curve 243 represents detected backscattered laserradiation containing substantially no Mie scattering, curve 243 may beused as a reference where Mie scattering has been eliminated. In oneexample, computer 156, therefore, uses curve 243 to remove the effectsof Mie scattering by aligning curves 241, 242 and 243 and by calculatinga ratio of the detected backscattered laser radiation to theoreticallypure Rayleigh scattering (the ratio of curves 241 and 242) which may beutilized to determine air density. Mie scattering effects are thenremoved by subtracting curve 243 from the calculated ratio of curves 241and 242. With Mie scattering essentially removed from the measurement,computer 156 more accurately determines air temperatures and airpressures.

FIGS. 6 and 7 show other exemplary graphs that may be used indetermining air pressure and air temperature. FIG. 6 illustrates a graph260 of electronic signals 163 and 167 of FIG. 2 respectively generatedby detectors 162 and 165 of FIG. 2. Graph 260 shows electronic signals163 and 167, that represent light intensity as a function of normalizedsignal strength (axis 261), versus frequency (axis 262). FIG. 7illustrates a graph 280 of electronic signals 158 and 155 (see FIG. 2)generated by detectors 153 and 154 respectively, representing lightintensity as a function of normalized signal strength (axis 281), versusfrequency (axis 282). These four light intensities (represented byelectronic signals 163, 167, 158 and 155) may be measured, over time,through transmission and collection of light corresponding to laserpulses, or they may be measured through transmission and collection oflight corresponding to a continuous wave laser whose frequency variescontinuously. In one example, a transmission frequency of laserradiation 142 of FIG. 2 generated by laser 141 at a certain PRF maysweep such that each laser pulse is emitted at a different frequency.Electronic signals 163 and 167 therefore illustrate how laser radiation142 of laser 141 may sweep in frequency across an absorption band 263 ofthe vapor filter 152. Illustratively, FIG. 6 shows one completefrequency sweep of laser radiation 142 generated by laser 141 anddetected by detectors 162 and 165. Similarly, electronic signals 155 and158 of FIG. 7 show detected signals of detectors 153 and 154 as laserradiation 142 of laser 141 performs a complete sweep in frequency acrossabsorption band 283 of vapor filter 152.

From signals 163 and 167, computer 156 may for example determine anormalized filter transmission, by dividing discrete points ofelectronic signal 167 by corresponding discrete points of signal 163.Similarly, computer 156 may determine a normalized atmospheric returnthough vapor filter 152 by dividing discrete points of signal 158 bycorresponding discrete points of signal 155. These discrete points,described herein, correspond to individual pulses of laser radiation142.

Using normalized calculations of filter transmission (e.g., from graph260) and the normalized calculations of atmospheric return (e.g., fromgraph 280), computer 156 determines relative optical efficiencies in thevapor filter 152.

In one embodiment, computer 156 determines optical transmission forvapor filter 152 using the frequency independent components of data fromgraph 260, FIG. 6 (there is substantially no change in amplitude forsignals 163 and 167 at frequencies greater in magnitude than ±18 GHzfrom 0 GHz illustrated at points 264, 265, 266 and 267). Computer 156therefore determines a ratio of optical transmission for vapor filter152 by calculating a ratio of signal 167 to signal 163, via frequencycorresponding points of the signals, for points representing frequenciesgreater in magnitude than ±18 GHz from 0 GHz.

Similarly, computer 156 determines a magnitude of intensity ofatmospheric-returned laser radiation received through vapor filter 152using the frequency independent parts of the data from graph 280, FIG. 7(there is substantially no change in amplitude for signals 155 and 158at frequencies greater in magnitude than ±18 GHz from 0 GHz illustratedat points 284, 285, 286 and 287). Computer 156 thereby determines aratio of atmospheric return with the laser power measurement bycalculating a ratio of signal 158 to signal 155 via frequencycorresponding points of the signals for points representing thefrequencies greater in magnitude than ±18 GHz from 0 GHz.

In one embodiment, computer 156 calculates a ratio of signal 158 tosignal 155 for frequencies between ±0.5 GHz (illustrated at points 288and 289). Such a frequency range includes substantially no Miescattering of laser radiation 142 for air 144; it thus corresponds tosubstantially pure Rayleigh scattering. Computer 156 thus compares aRayleigh to Mie scattering strength based upon the ratio of signal 158to signal 155. Computer 156 determines Rayleigh to Mie scatteringstrength by comparing a ratio of signal 158 to signal 155 at frequenciesbetween ±0.5 GHz to the ratio of signal 158 to signal 155 at frequenciesgreater than ±18 GHz from 0 GHz. In one embodiment, computer 156performs similar calculations for “non-scattered” laser radiation 142(e.g., component 143B of FIG. 2) based on data illustrated in FIG. 6using points 268 and 269. Such a process is further described in FIG. 8.

Ratios determined for the non-scattered laser radiation 142 and for thescattered laser radiation 142 may be used in tandem to numericallycalculate Laser-Rayleigh-Filter convolution (e.g., LRF(v)) from data.The Laser-Rayleigh-Filter convolution is in turn compared to a look uptable of theoretical Laser-Rayleigh-Filter convolution values todetermine temperature and pressure.

FIG. 8 shows a flowchart of one exemplary methodical operation 400 of anOADS. Method 400 may be partially or fully performed by computer 156 ofOADS 140; computer 156 may receive operating instructions from softwareand/or firmware. A laser (e.g., laser 141 of FIG. 2) sweeps laserradiation across a predetermined frequency spectrum, in step 401. Thelaser may sweep the laser radiation across a frequency range of about±20 GHz by transmitting laser radiation at a certain PRF (or it maysweep frequency continuously, as discussed in connection with FIG. 2above). In one embodiment, the PRF is about 1 kHz, with a pulse widthbetween about 50 ns and 100 ns, and a swept frequency range is centeredabout a frequency corresponding to a peak absorption frequency (e.g.,260 nm) of a filter (e.g., vapor filter 152, FIG. 2).

Laser radiation is typically split into four distinct paths such thatthe laser radiation may be detected as four different inputs, in step402. These four paths of laser radiation correspond to: 1) laserradiation transmitted by the laser (e.g., component 159 of FIG. 2); 2)laser radiation transmitted by the laser through the filter (e.g.,component 164 of FIG. 2); 3) laser radiation transmitted by the laserinto the air and backscattered (e.g., component 150A of FIG. 2); and 4)laser radiation transmitted by the laser into the air and backscatteredthrough the filter (e.g., component 157 of FIG. 2). For simplicity,these components are hereinafter referred to as: 1) unfiltered laserradiation; 2) filtered laser radiation; 3) unfiltered backscatteredlaser radiation or unfiltered scattered laser radiation; and 4) filteredbackscattered laser radiation or filtered scattered laser radiation.

After detecting the four components of laser radiation, a computer(e.g., computer 156, FIG. 2), determines normalized filter transmissionof the vapor filter, in step 403. For example, the computer, in oneembodiment, processes the unfiltered laser radiation and filtered laserradiation by dividing the magnitude of the filtered laser radiation bythe magnitude of the unfiltered laser radiation. In one embodiment, thedivision is performed on a pulse by pulse basis, where dividedmagnitudes of the pulses have corresponding frequencies.

The computer also determines, in one embodiment, a normalizedatmospheric return of the laser radiation, in step 404. For example, thecomputer may process the filtered backscattered laser radiation andunfiltered backscattered laser radiation by dividing the magnitude ofthe filtered backscattered laser radiation by the magnitude of theunfiltered backscattered laser radiation. Again, in one embodiment,division is performed on a pulse by pulse basis, where dividedmagnitudes of the pulses have corresponding frequencies.

Once normalized filter transmission and normalized atmospheric return ofthe laser radiation are determined, the computer determines signalstrengths for each of the filter transmission and the atmosphericreturn. For example, the computer determines the optical transmissionthrough the filter by calculating a ratio of the filtered laserradiation to the unfiltered laser radiation at particular frequencyranges, in steps 405 and 407. The computer similarly determines theatmospheric return (scattering) signal strength through the filter bycalculating a ratio of the filtered backscattered laser radiation to theunfiltered laser radiation at particular frequency ranges, in steps 406and 408.

The computer also determines a signal strength ratio for the normalizedfilter transmission by dividing filtered laser radiation by unfilteredlaser radiation, again on a pulse by pulse basis, at frequencies greaterin magnitude than about ±18 GHz about the peak absorption frequency, instep 407. The computer further determines a signal strength ratio forthe normalized filter transmission by dividing filtered laser radiationby unfiltered laser radiation on a pulse by pulse basis at frequenciesbetween about ±0.5 GHz, in step 405. These signal strengthdeterminations correspond to frequency ranges where Mie scattering(e.g., ±18 GHz) and Rayleigh scattering (e.g., ±0.5 GHz) are mostprevalent, and are thus useful when combined with similar signalstrength determinations for the normalized atmospheric return. Thecomputer determines a Mie scattering signal strength ratio for thenormalized atmospheric return of the laser radiation by dividingfiltered backscattered laser radiation by unfiltered backscattered laserradiation, again on a pulse by pulse basis, at frequencies greater inmagnitude than about ±18 GHz about the peak absorption frequency, instep 408. The computer also determines a Rayleigh scattering signalstrength ratio for the normalized atmospheric return of the laserradiation by dividing filtered scattered laser radiation by unfilteredbackscattered laser radiation on a pulse by pulse basis at frequenciesbetween about ±0.5 GHz in step 406.

With signal optical transmission for the filter and signal strengths forboth Rayleigh scattering and Mie scattering determined, the computerdetermines a Rayleigh laser filter convolution in step 409. For example,the computer, in one embodiment, performs a convolution of the opticaltransmission with the Rayleigh and Mie scattering signal strengthscorresponding to the frequency ranges for Rayleigh and Mie scattering of±0.5 GHz and ±18 GHz, respectively. The computer then accesses a lookuptable, such as lookup table 170 of FIG. 2, that has theoretical Rayleighlaser filter convolution values to determine temperature and pressure ofthe air, in step 410.

It is also possible to calculate a convolution of a measured filterfunction with a theoretical Rayleigh-Brillouin return (Rayleigh lineshape), and directly compare the convolution with filtered scatteredlaser radiation. This allows calculation of atmospheric parameterswithout calculating a deconvolution of the Rayleigh-Brillouin signal,reducing the complexity of real-time calculations required to determinethe atmospheric parameters. In particular, ratios of measured signalsmay be compared directly to theoretical ratios of a Rayleigh line shapeconvolved with measured filter functions to allow self calibratingmeasurements. For example, signal strength variations across datagathering channels and power of scattered laser radiation may beinherently normalized when such ratios are used. Certain ratios ofmeasured data at laser frequencies that lie within filtered bands ofband-stop filters (e.g., absorption features of an atomic vapor cell, orequivalent features of other filters, as discussed above) may be usefulfor determining temperature and pressure, since Mie scattering iseliminated from the measured data. Calculation of convolutions mayrepresent a lower computational burden on a computer (e.g., computer 156of OADS 140) as compared to calculating deconvolutions of measured datainto and out of a Rayleigh-Brillouin representation.

For example, filtered scattered laser radiation data in a signal channelmay characterized by the equation:S _(S)(v)=P _(L) T _(L) T _(R) E _(S) D _(S) ∫∫dv ₁ dv _(r) L(v_(laser))F(v ₁ −v)(rR(v−(v _(r) −Δv _(D)))+mM(v−(v _(r) −Δv _(D))))  (Eq. 24)where parameters are as previously defined, and a subscript 1 indicatesa measurement frequency 1. Eq. 24 may be simplified by using thenotation LFR(v) for the convolution of laser, filter function andRayleigh return, as defined above, and a similar notation LFM(v) for aconvolution of laser, filter function, and Mie scattering return:S _(S)(v ₁)=P _(L1) T _(L1) T _(R1) E _(S1) D _(S1) [rLFR(v ₁ +mLFM(v₁)]  (Eq. 25)

If frequency I is located in a filter absorption band, the Miescattering term is effectively eliminated, yielding:S _(S)(v ₁)=P _(L1) T _(L1) T _(R1) E _(S1) D _(S1) rLFR(v ₁)  (Eq. 26)

Forming a ratio of a signal obtained at frequency 1 with a signalobtained at another frequency 2 in another filter absorption bandyields: $\begin{matrix}{\frac{S_{S}\left( v_{1} \right)}{S_{N}\left( v_{2} \right)} = {{\left\lbrack \frac{P_{La}L_{La}T_{Ra}E_{Sa}D_{Sa}}{P_{Lb}L_{Lb}T_{Rb}E_{Sb}D_{Sb}} \right\rbrack\left\lbrack \frac{r_{a}\left( {r_{b} + m_{b}} \right)}{r_{b}\left( {r_{a} + m_{a}} \right)} \right\rbrack}\left\lbrack \frac{{LFR}\left( v_{1} \right)}{{LFR}\left( v_{2} \right)} \right\rbrack}} & \left( {{Eq}.\quad 27} \right)\end{matrix}$

where frequency 1 is measured at time a and frequency 2 is measured attime b. If times a and b are close enough to each other that noatmospheric changes occur between time a and time b (or if measurementsare interspersed in such a way that average values of parameters such asP, L, T, E, D and r are identical over a time span of the measurements)then the ratio of Eq. 27 simplifies further to: $\begin{matrix}{\frac{S_{S}\left( v_{1} \right)}{S_{N}\left( v_{2} \right)} = \frac{{LFR}\left( v_{1} \right)}{{LFR}\left( v_{2} \right)}} & \left( {{Eq}.\quad 28} \right)\end{matrix}$

In one embodiment, a lookup table stores temperature and pressure pairsthat correspond with two of the measurement ratios defined in Eq. 28.The two ratios essentially define two equations with two unknowns (i.e.,a single such ratio may not determine both pressure and temperature).Data may also be taken in more than three filtered bands, yielding morethan two of the Eq. 28 ratios; when more than two such ratios areavailable, multiple values of temperature and pressure may be determinedthat may be averaged or used in “best fit” methods to improvetemperature and pressure determination in a noisy measurementenvironment.

In an embodiment, theoretical Rayleigh line shapes corresponding totemperature and pressure combinations are stored in a lookup table. Areference curve is calculated by obtaining a Rayleigh line shapecorresponding to an estimated temperature and pressure from the lookuptable and convolving the Rayleigh line shape with a normalizedatmospheric return curve. Values of LFR(v) at absorption feature maximamay then be determined from the reference curve and used to determineone or more air parameters (e.g. temperature and/or pressure) using Eq.28.

Calculating convolution of the measured filter function with thetheoretical Rayleigh line shape also enables utilization of curvefitting routines to map the convolved curves to true temperature andpressure conditions, such that the deconvolution calculations suggestedby the Tenti, Boley and Desai paper above are not required. An OADS maystore a pre-compiled table of stored curve shapes that are generated bymodeling large databases of known temperature and pressure values (e.g.,the table may be stored in computer 156 of OADS 140). As measurementsare taken, data curves may be generated from measured data, andcurve-fitting routines may be used to compare the data curves to thestored curve shapes to derive true temperature and pressure. Theutilization of curve-fitting routines may also have less sensitivity tonoisy data, as compared to deconvolution calculations, making thedetermination of true temperature and pressure more robust.

FIG. 9 is a flowchart showing one exemplary method of operation 450 ofan OADS, which may be used to calculate one or more air parameters.Method 450 may be partially or fully performed by computer 156 of OADS140; computer 156 may receive operating instructions from softwareand/or firmware.

In an embodiment of step 460, a laser (e.g., laser 141 of FIG. 2) sweepslaser radiation across a predetermined frequency range (swept frequencyrange) that is centered about a deep absorption line of a filter. Thelaser may sweep the laser radiation across a swept frequency range ofabout ±20 GHz by transmitting the laser radiation at a certain PRF, orby sweeping the frequency of a continuous wave laser. In one embodiment,a PRF is about 1 kHz, with a pulse width between about 50 ns and 100 ns,and the swept frequency range is centered about a frequencycorresponding to a peak absorption frequency (e.g., 260 nm) of a filter(e.g., vapor filter 152, FIG. 2, or an interference filter, a fiberBragg grating filter, a dichroic filter or a Rugate filter). In anembodiment, the swept frequency range includes frequencies correspondingto at least two absorption features of at least one band stop filter. Inanother embodiment, the swept frequency range includes frequenciescorresponding to at least three absorption features of at least one bandstop filter.

Step 462 detects laser radiation corresponding to filtered scatteredlaser radiation (e.g. component 157 of FIG. 2), filtered laser radiation(e.g. component 164 of FIG. 2), and unfiltered laser radiation (e.g.component 159 of FIG. 2) at each frequency; each step 460 and 462 is forexample performed for each laser pulse in the swept frequency range.

Step 464 determines a normalized filter transmission curve by dividing amagnitude of filtered laser radiation by a magnitude of unfiltered laserradiation for each pulse in the swept frequency range; step 466determines a normalized atmospheric return curve by dividing a magnitudeof filtered scattered laser radiation by a magnitude of unfiltered laserradiation for each pulse in the swept frequency range. It will beappreciated that since the data required for the calculations in steps464 and 466 are collected by the operation of steps 460 and 462, steps464 and 466 may be done in any order or in parallel.

Step 468 calculates a Doppler shift Δv_(D) that is a frequency shiftbetween the normalized filter transmission curve (calculated in step464) and the normalized atmospheric return curve (calculated in step466), then calculates a local radial wind velocity v_(R) using Eq. 1above. As was stated above, a band stop filter may have a plurality ofabsorption features; consequently, a plurality of Doppler shift Δv_(D)and radial wind velocity v_(R) calculations may be calculated in step468.

Step 470 utilizes only normalized atmospheric return curve magnitudevalues (calculated in step 466) within three or more specific filterabsorption bands to form two or more normalized atmospheric returnratios (actual ratios). For example, if atmospheric return data isderived for frequencies 1, 2, and 3 (at times that are close enoughtogether, as discussed with reference to Eq. 28 above), then two ratiosmay be formed using one of the frequencies as a baseline (denominator),such as$\frac{S_{S}\left( v_{1} \right)}{S_{S}\left( v_{2} \right)}\quad{and}\quad{\frac{S_{S}\left( v_{3} \right)}{S_{S}\left( v_{2} \right)}.}$Atmospheric return curve magnitude values corresponding to absorptionfeature maxima of one or more band stop filters may be used. Oneatmospheric return ratio (actual ratio) may be determined if for exampleonly one air parameter (e.g. pressure or temperature) is to becalculated.

Step 472 obtains theoretical temperature and pressure data from a lookuptable of normalized filter transmission convolved with theoreticallyderived Rayleigh line shapes, at the frequencies utilized in step 470.One or more air parameters (e.g. temperature and/or pressure) are thenestimated. A Rayleigh line shape corresponding to the estimated one ormore air parameters is for example obtained from a lookup table. Areference curve is then calculated by convolving the Rayleigh line shapewith the normalized filter transmission curve from step 464.

In step 474, ratios corresponding to the ratios formed in step 470 areformed from magnitude values of the reference curve calculated in step472. The ratios formed in step 474 may be referred to as referenceratios.

In step 476, one or more air parameters (e.g. temperature and pressure)are determined. An error corresponding to the differences between theone or more actual ratios and the corresponding one or more referenceratios may be calculated: if the error is within an acceptable range,the estimated one or more air parameters (corresponding to the Rayleighline shape) are published as the actual one or more air parameters; butif the error is not within an acceptable range, steps 472, 474, and 476are repeated with one or more different estimated air parameters untilthe error is within an acceptable range.

The error of step 476 may be calculated using a least mean square erroralgorithm. Steps 470, 474, and 476 may be optional; the normalizedatmospheric return curve calculated in step 466 is for examplecorrelated to the reference curve calculated in step 472 using curvefitting routines.

Certain advantages of embodiments described above may include:

(1) Obtaining accurate computations of various air parameters, such asair speed, air temperature and air pressure, substantially regardless ofaltitude and/or Mie scattering;

(2) Obtaining a system that accurately performs in a variety ofvibrational environments;

(3) Obtaining an ability to determine temperature and pressure within aparticular region of atmosphere without a prior knowledge of theatmosphere;

(4) Reducing need for on-aircraft system calibrations and system healthchecks, as compared to existing systems;

(5) Providing robustness with respect to high vibration environments;

(6) Obtaining faster calculations and/or reduced computationalrequirements placed on aircraft computers, as compared to existingsystems;

(7) Ability to accurately calculate velocity in environments withchanging temperature and/or pressure; and/or

(8) Ability to accurately calculate one or more air parameters (e.g.velocity, temperature, and/or pressure) without precise control of laserfrequency.

Since certain changes may be made in the above methods and systemswithout departing from the scope of the disclosure herein, it isintended that all matter contained in the above description or shown inthe accompanying drawings be interpreted as illustrative and not in alimiting sense. By way of example, those skilled in the art shouldappreciate that the OADS and the OADS transceivers, as described herein,may be constructed, connected, arranged, and/or combined in ways thatare equivalent to what is shown.

1. A method for remotely sensing air outside a moving aircraft,comprising generating laser radiation within a swept frequency range;projecting a portion of the laser radiation from the aircraft into theair to induce scattered laser radiation; detecting filtered scatteredlaser radiation, filtered laser radiation, and unfiltered laserradiation; determining at least one actual ratio from data correspondingto the filtered scattered laser radiation and the unfiltered laserradiation; and determining one or more air parameters by correlating theactual ratio to at least one reference ratio.
 2. The method of claim 1,the step of detecting comprising utilizing at least one band stopfilter.
 3. The method of claim 2, wherein the at least one band stopfilter is selected from the group consisting of a fixed frequency atomicvapor filter, an interference filter, a dichroic filter, a fiber Bragggrating filter, a Rugate filter, and combinations thereof.
 4. The methodof claim 1, the step of generating comprising generating the laserradiation in pulses, each pulse having a different peak frequency withinthe swept frequency range.
 5. The method of claim 1, the step ofgenerating comprising sweeping a frequency of the laser continuouslyacross the swept frequency range.
 6. The method of claim 1, wherein theswept frequency range includes frequencies corresponding to at least twoabsorption features of at least one band stop filter.
 7. The method ofclaim 1 further comprising calculating a normalized filter transmissioncurve and a normalized atmospheric return curve from the filteredscattered laser radiation, the filtered laser radiation and theunfiltered laser radiation.
 8. The method of claim 7 further comprisingdetermining a Doppler line shift and a radial wind velocity from afrequency shift between the normalized filter transmission curve and thenormalized atmospheric return curve.
 9. The method of claim 8 furthercomprising determining a plurality of Doppler line shifts, each Dopplerline shift corresponding to an absorption feature of at least one bandstop filter.
 10. The method of claim 9 further comprising determining aradial wind velocity from the plurality of Doppler line shifts.
 11. Themethod of claim 1, wherein the air parameters further comprisetemperature and pressure.
 12. A method for remotely sensing air outsidea moving aircraft, comprising: generating laser radiation within a sweptfrequency range, the swept frequency range including at least twoabsorption features of at least one band stop filter; projecting aportion of the laser radiation from the aircraft into the air to inducescattered radiation; detecting filtered scattered laser radiation,filtered laser radiation, and unfiltered laser radiation; determining anormalized atmospheric return curve from the filtered scattered laserradiation and the unfiltered laser radiation; determining a normalizedfilter transmission curve from the filtered laser radiation and theunfiltered laser radiation; determining at least one actual ratio fromthe normalized atmospheric return curve; determining at least onereference ratio from the normalized filter transmission curve and aRayleigh line shape corresponding to one or more estimated airparameters; and determining one or more air parameters by correlatingthe at least one actual ratio to the at least one reference ratio. 13.The method of claim 12, wherein the step of determining at least oneactual ratio further comprises: determining at least two normalizedatmospheric return curve magnitude values, each normalized atmosphericreturn curve magnitude value corresponding to an absorption feature ofthe at least one band stop filter; and determining the at least oneactual ratio from the normalized atmospheric return curve magnitudevalues.
 14. The method of claim 13, wherein the step of determining theat least one reference ratio further comprises: determining a referencecurve by convolving the Rayleigh line shape with the normalizedatmospheric return curve; determining reference curve magnitude valuescorresponding to each frequency of the normalized atmospheric returncurve magnitude values; and determining a reference ratio correspondingto each actual ratio, each reference ratio including two reference curvemagnitude values.
 15. The method of claim 12, wherein the step ofdetermining one or more air parameters further comprises: determining anerror corresponding to differences between the at least one actual ratioand the at least one reference ratio; designating the one or moreestimated air parameters as actual air parameters if the error is withinan acceptable range; and repeating the step of determining at least onereference ratio using different estimated air parameters if the error isnot within the acceptable range.
 16. The method of claim 12, wherein theair parameters further comprise temperature and pressure.
 17. A methodfor remotely sensing air outside a moving aircraft, comprisinggenerating laser radiation within a swept frequency range; projecting aportion of the laser radiation from the aircraft into the air to inducescattered radiation; detecting filtered scattered laser radiation,filtered laser radiation, unfiltered scattered laser radiation, andunfiltered laser radiation; determining a normalized filter transmissionand a normalized atmospheric return from the filtered scattered laserradiation, filtered laser radiation, unfiltered scattered laserradiation, and unfiltered laser radiation; and determining a pluralityDoppler line shifts and a plurality of radial wind velocities from aplurality of frequency shifts between the normalized filter transmissionand the normalized atmospheric return, each frequency shiftcorresponding to an absorption feature of at least one band stop filter.18. A system for sensing of air outside a moving aircraft, comprising:at least one laser for generating laser radiation; at least onetransceiver for projecting the laser radiation to the air and forreceiving scattered laser radiation from the air; at least one band stopfilter selected from the group consisting of a fixed frequency atomicvapor filter, an interference filter, a dichroic filter, a fiber Bragggrating filter, a Rugate filter, and combinations thereof; and acomputer for controlling the laser and for processing signals from thetransceiver to determine one or more air parameters based on thescattered laser radiation.
 19. The system of claim 18, wherein thetransceiver is configured to detect signals corresponding to filteredscattered laser radiation, filtered laser radiation, and unfilteredlaser radiation.
 20. The system of claim 19, wherein the computer isconfigured to determine a normalized filter transmission curve and anormalized atmospheric return curve from the filtered scattered laserradiation, the filtered laser radiation, and the unfiltered laserradiation.
 21. The system of claim 20, wherein the computer isconfigured to determine a Doppler line shift and a radial wind velocityfrom a frequency shift between the normalized filter transmission curveand the normalized atmospheric return curve.
 22. The system of claim 21wherein the computer is configured to determine a plurality of Dopplerline shifts corresponding to a plurality of absorption features of theat least one band stop filter.
 23. The system of claim 20, wherein thecomputer controls the laser to generate laser radiation that defines aswept frequency range corresponding to at least two absorption featuresof the at least one band stop filter.
 24. The system of claim 23,wherein the computer is configured to utilize the normalized filtertransmission curve to form at least one actual ratio and to determineair temperature and pressure from the at least one actual ratio and atleast one reference ratio.
 25. A software product comprisinginstructions, stored on computer-readable media, wherein theinstructions, when executed by a computer, perform the steps forremotely sensing air outside a moving aircraft, comprising: instructionsfor generating laser radiation within a swept frequency range;instructions for determining a normalized atmospheric return curve fromfiltered scattered laser radiation and unfiltered laser radiation;instructions for determining a normalized filter transmission curve fromfiltered laser radiation and the unfiltered laser radiation;instructions for determining at least one actual ratio from thenormalized atmospheric return curve; instructions for determining atleast one reference ratio from the normalized filter transmission curveand a Rayleigh line shape corresponding to one or more estimated airparameters; and instructions for determining one or more air parametersby correlating the at least one actual ratio to the at least onereference ratio.
 26. The software product of claim 25, the instructionsfor determining at least one actual ratio comprising: instructions fordetermining at least two normalized atmospheric return curve magnitudevalues, each normalized atmospheric return curve magnitude valuecorresponding to an absorption feature of at least one band stop filter;and instructions for determining the at least one actual ratio from thenormalized atmospheric return curve magnitude values.
 27. The softwareproduct of claim 26, the instructions for determining at least onereference ratio comprising: instructions for determining a referencecurve by convolving the Rayleigh line shape with the normalizedatmospheric return curve; instructions for determining reference curvemagnitude values corresponding to each frequency of the normalizedatmospheric return curve magnitude values; and instructions fordetermining a reference ratio corresponding to each actual ratio, eachreference ratio including two reference curve magnitude values.
 28. Thesoftware product of claim 25, the instructions for determining one ormore air parameters comprising: instructions for determining an errorcorresponding to differences between the at least one actual ratio andthe at least one reference ratio; instructions for designating the oneor more estimated air parameters as actual air parameters if the erroris within an acceptable range; and instructions for repeating the stepof determining at least one reference ratio using different estimatedair parameters if the error is not within the acceptable range.